Prosthiostomum siphunculus (Delle Chiaje, 1822) Lang, 1884

Gammoudi, Mehrez & Tekaya, Saïda, 2017, Suborders Acotylea and Cotylea (Polycladida): Study on morphological, ecological and reproductive features of some representative species from Tunisian coasts (Mediterranean), Ecologica Montenegrina 10, pp. 58-70 : 67

Prosthiostomum siphunculus (Delle Chiaje, 1822) Lang, 1884

Morphological features. The body is elongated rounded anteriorly and pointed posteriorly. The length can reach 28 mm. Dorsal face is smooth and shows clearly the tubular pharynx. The tentacles are lacking ( Fig. 1H ). Marginal, cerebral and tentacular eyes are present. Ventral face is transparent and shows tubular pharynx, vasa deferentia and uteri. A developed true sucker is present behind genital pores ( Fig. 3C ).

Reproductive features. Testes are mainly ventral and ovaries are dispersed dorsally. A pair of free prostatic vesicles of free-type is present. The unique seminal vesicle receives the two prostatic canals and communicates with the ejaculatory duct which leads to a penis papillae armed with a stylet. The two gonopores are separated. The vagina shows a widening, the cement pouch and then curve dorsally where it receives the uteri.

Developmental model. The development is indirect and a ciliated Müller's larva hatchs ( Fig. 5C, D ). It is provided with eight lobes.

Associated fauna. Specimens were found to be associated with the sea urchin Paracentrotus lividus (Lamarck, 1816) .

Figures

Figure 1. Anterior dorsal region in A: Echinoplana celerrima. See the tentacular (te) and cerebral eyes (ce) forming two paramedian bands, scale bar: 0.5 mm. B: Letoplana mediterranea. See clusters of tentacular (te) and cerebral eyes (ce), scale bar: 0.5 mm. C: Discocelis tigrina. Note the presence of cerebral and tentacular eyes clusters, scale bar: 0.5 mm. D: Imogine mediterranea. Note the presence of two everted nuchal tentacles (arrows), scale bar: 0.5 mm. E: Yungia aurantiaca. See the two marginal tentacles (arrows) and cerebral eyes spots (arrowhead), scale bar: 1 mm. F: Thysanozoon brocchii. Dorsal faces provided with papillae and developed marginal tentacles (arrows), scale bar: 1 mm. G: Prostheceraeus moseleyi. Note the presence of marginal tentacles (arrows) and yellow marginal line, scale bar: mm. H: Prosthiostomum siphunculus. The cerebral eyes (ce) are not included in the semi circle formed by marginal eyes (me), scale bar: 1 mm.

Figure 3. Ovaries in polyclads A: Section through ovary of Echinoplana celerrima showing a ventral growing zone (grz) and a dorsal germinative zone (gmz), scale bar: 0.01 mm. B: Section through ovary of the cotylean Thysanozoon brocchii showing a ventral germinative zone (gmz) and a dorsal growing zone (grz), scale bar: 0.01 mm.

Figure 5. Hatched juveniles and larvae in polyclads A: A twelve eyed juvenile of Leptoplana mediterranea, scale bar: 0.05 mm B: Götte’s larva of Imogine mediterranea: lateral view, scale bar: 0.03 mm C: Müller’s larva of Prosthiostomum siphunculus: lateral view, scale bar: 0.03 mm D: Müller’s larva of Prosthiostomum siphunculus: apical view showing the three eyes (arrowheads), scale bar: 0.03 mm.